1. Field of the Invention
The present invention relates to a system and a method for diagnosing deterioration of a catalyst installed in the exhaust stream of an internal combustion engine, and more particularly to a system and a method for diagnosing deterioration of a NOx-occluded type catalyst in the exhaust stream.
2. Description of the Prior Art
As is known, the NOx-occluded type catalyst has such a performance that when the exhaust gas from the engine shows a higher air/fuel ratio (viz., leaner), the catalyst occludes NOx in the exhaust gas, while when the exhaust gas shows a stoichiometric and/or lower air/fuel ratio (viz., richer), the catalyst releases NOx therefrom allowing reduction (or deoxidation) of NOx in the exhaust gas with the aid of HC and CO.
One of the systems for diagnosing deterioration of such NOx-occluded type catalyst is shown in Japanese Patent 2586739. In this system, under operation of the engine, the amount of NOx occluded by the NOx-occluded type catalyst is estimated and when the estimated amount of NOx exceeds a predetermined amount, the concentration of NOx at a position downstream of the catalyst is detected to carry out the deterioration diagnosis of the catalyst. That is, in the system, judgement as to deterioration of the catalyst is effected by checking the overflowed amount of NOx from the catalyst with reference to a predetermined value.
For ease of description, in the following, the NOx-occluded type catalyst will be referred to as NOx-occluded catalyst, and an air/fuel ratio higher or lower than a stoichiometric air/fuel ratio (viz., 14.6/1) will be referred to a leaner or richer air/fuel ratio.